The design and use of filter circuitry for eliminating a signal of undesired frequency is well known. It is also known that these filters can be fabricated from ceramic materials having one or more resonators formed therein.
Many conventional ceramic block filters are comprised of parallelepiped shaped blocks of dielectric material through which many holes may extend from one surface to an opposite surface. Often, these filters use embedded features on the top surface in order to obtain the desired frequency characteristics of the filter.
It is well known that the top end of the resonators in a block filter have strong electric fields radiating therefrom which may adversely effect circuitry surrounding the filter in a radio or other communication device or apparatus. These radiating electric fields may also adversely effect the performance of the filter itself. In conventional filters, electric field radiation is minimized by enclosing the filter in a grounded metal housing.
Electric field radiation may also be reduced by enclosing or otherwise confining the top surface of the filter in a metal grounded bracket, which is typically soldered to the exterior sides of the block filter. Another alternative involves the use of L-Shaped stamped metal shields which are mounted to a side surface of the filter and wrap around to protect the top surfaces of the filter.
Unfortunately, the use of L-Shaped stamped metal shields presents a variety of problems during the manufacturing stage of the shielded filter and additional problems when the filter is placed onto a circuit board in communication devices. Problems include the areas of soldering, adhesion, parallelism, coplanarity, size, weight, and the number of processing steps. One significant problem for a manufacturer which uses the filter is the fact that the bottom edge of the L-Shaped stamped metal shield must be properly soldered to the circuit board to assure proper grounding of the ceramic filter. This problem is compounded by the variation in the ceramic block dimensions due to filter manufacturing process tolerances, even though the shield dimensions can be well controlled.
Another problem is encountered when low-profile components are desired. As the size of the filter block decreases, the thickness of the shield, and more significantly, the distance which the shield rests atop the filter block, becomes a greater contributor to the overall size of the filter. As the filter block size decreases, even the attachment of a metallic shield to a side surface of the block may add an undesirable "height above the circuit board" to the filter.
FIG. 1A shows a ceramic block filter with an attached external shield in accordance with the prior art. Referring to FIG. 1A, a dielectric block of ceramic 102 has a metallic shield 104 attached to the top surface thereof. It should be noted that the shield rests a predetermined distance above the ceramic block 102, adding substantial height to the filter component.
FIG. 1B and FIG. 1C show two different techniques for attaching the external shields 104 to the block of dielectric ceramic 102 and the circuit board 106 in accordance with the prior art. In FIG. 1B, the metallic shield 104 is attached directly to the block of dielectric ceramic 102 whereas in FIG. 1C, the metallic shield 104 is attached to both the block of dielectric ceramic 102 as well as to the circuit board 106. In both instances, the metallic shield 104 adds substantial size and volume to the overall filter component dimensions.
It would be considered an improvement in the art to provide a ceramic filter with a recessed shield design which is entirely self-contained and can be attached directly to the conductive metallization layer on the top surface of the filter, while also providing the advantages of a smaller-sized, rugged, compact filter component which is particularly well suited for large scale and automated manufacturing processes and operations and which provides for easier fixturing, assembly, and testing.